1. Field of the Invention
The present invention relates to a semiconductor device in which an active layer is formed of a crystalline semiconductor film having a crystal structure and a method of forming the same. Particularly, the invention relates to a thin film transistor in which an active region is formed of a crystalline semiconductor film, a semiconductor device such as an integrated circuit using the thin film transistor and a method of forming fabricating the semiconductor device. Further, in the specification, a semiconductor device refers to general device functioning by utilizing a semiconductor characteristic, including, for example, a semiconductor integrated circuit, an active matrix type display device, and other electronic device such as an electronic device mounted with a semiconductor integrated circuit or an active matrix type display device.
2. Description of the Related Art
In developing an active matrix type display which is a kind of a flat panel display, there has been developed a technology of forming a thin film transistor (hereinafter, described as TFT) using a crystalline semiconductor film having a crystal structure for an active layer as an element of a pixel portion or a transistor of an integrated circuit for driving the pixel portion and there is realized a monolithic type display integrated with an integrated circuit of a pixel portion and a drive circuit necessary for driving the pixel portion on one glass substrate or quartz substrate. There have been sold various commodities such as a notebook type personal computer or a portable telephone mounted with an active matrix type liquid crystal display device formed by TFT using a polycrystal silicon film produced by crystallizing an amorphous silicon film.
There is requested TFT having fast operation speed, that is, high field effect mobility in order to realize higher pixel formation and higher miniaturization of a monolithic type of an active matrix type display. In order to achieve high field effect mobility, it is necessary to form TFT by using a crystalline silicon film produced by crystallizing an amorphous silicon film and for that purpose, an intensive research has been carried out on a technology of crystallizing a silicon film. As the crystallizing technology, there is known a method of heating an amorphous semiconductor film by a heating furnace or RTA apparatus for solid phase growth or a method of crystallizing an amorphous film by heating the amorphous film by irradiating laser beam.
Although in order to increase the field effect mobility of TFT, carriers may be moved smoothly by a channel without being scattered, in the case of TFT using a crystalline silicon film currently reduced to practice, there are many grain boundaries in the channel and therefore, the field effect mobility of TFT cannot be made higher than that of a transistor using a silicon wafer.
Hence, it has been tried to enlarge a crystal grain of a semiconductor of the channel to bring the field effect mobility of TFT close to that of a transistor of a single crystal silicon wafer. Because by enlarging the crystal grain, a number of crystal grain boundaries of a semiconductor at a channel of TFT is reduced and therefore, a probability that carriers are scattered by crystal grain boundaries can be reduced.
Further, although it is known that easiness of flowing carriers in a semiconductor differs also by crystal orientation, according to the conventional crystallizing technology, it is very difficult to align crystal orientation in the direction of carriers flowing in the channel. According to the above-described conventional crystallizing technology, crystals grow with crystal nuclei accidentally produced in an amorphous silicon film as seeds and therefore, the crystal grain boundaries cannot be eliminated at all, further, it is very difficult to control positions of crystal grain boundaries or crystal orientation.
That the positions of the crystal grain boundaries and the crystal orientation cannot be aligned in this way, is that the crystal structure of the crystallized silicon film differs at respective locations and therefore, even when TFT is formed by using the same silicon film, there is constituted one of causes of varying characteristics of TFT at respective locations.
Further, as TFT, there is also requested a characteristic that as a switching element, a threshold voltage value is small and a sub threshold value (S value) is small. It is known that in order to improve such a characteristic, a film thickness of the semiconductor film of the channel portion is thinned. This is because a characteristic of a sub threshold region on I-V characteristic is improved since when the semiconductor film of the channel portion is thinned, spread of a depletion layer (channel) in a film thickness direction (vertical direction) is restrained.
In the case of TFT using a polycrystal silicon film, normally, it is preferable to thin a channel forming region to be equal to or smaller than about 60 nm. However, in the case of crystallization by solid phase growth using a heating furnace or the like, when the film thickness is thinned, there is a case that it is difficult to increase a crystal grain size.
Further, also in the case of crystallization by continuous wave laser beam such as YAG laser or pulse laser beam such as excimer laser, similar to the case of solid phase growth using a heating furnace or the like, it is difficult to thin the film thickness of the amorphous silicon film. This is because there is posed a new problem that abrasion is liable to be caused and a margin of optimum energy is narrowed and in the case of continuous wave laser beam, unless the film thickness is equal to or larger than 60 nm in the case of continuous wave laser beam or 50 nm in the case of excimer laser, it is difficult to set optimum energy and therefore, it is difficult to reproducibly carry out crystallization.
Further, conventionally in order to thin the thickness of the semiconductor film of the channel portion, there is known a method of using thermal oxidation, however, according to the method, only a limited substrate having heat resistance, such as a quartz substrate or a silicon wafer, can be used.
As described above, according to conventional crystallization by laser annealing or crystallization of an amorphous silicon by solid phase growth by using an electric furnace, the crystallization is derived from crystal growth from accidentally produced crystal nuclei, a position of generating and a density of generating crystal nuclei cannot be controlled, further, orientations of crystalline planes of a semiconductor film cannot be controlled.